In a switching power supply as a voltage converter, problems such as generation of heat and noise are exposed due to tendency of increasing in electric current and increasing in switching frequency. As measures against noise generated due to switching (switching noise), the following method is known. In a known method, an input voltage (e.g., a gate voltage of a MOS transistor) of a switching element is decreased, and a voltage change amount of an output voltage of the switching element (e.g., a source voltage of the MOS transistor) is decreased to restrict switching noise. However, in the above-described method, a rise time (turn-on period) of the switching element increases. Thus, a power consumption and a heat generation amount increase.
Thus, a method of reducing switching noise while restricting generation of heat is required. JP-A-2011-142815 discloses a technique in which an input voltage of a switching element is controlled with fixing a rise time so as to change the voltage change amount during the turn-on period. Specifically, when the switching element is turned on, the voltage change amount is large, that is, the output voltage drastically increases until the output voltage reaches a predetermined threshold value. After the output voltage of the switching element reaches the threshold value, the voltage change amount is small, that is, the output voltage gently increases. By the above-described technique, the output voltage gently increases after the output voltage reaches the threshold value. Thus, the voltage change amount at the end of turning-on decreases, generation of overshoot can be restricted, and noise can be reduced.
In the switching power supply, switching noise is caused mainly by the following reasons rather than the overshoot. In a boost (step-up) switching power supply, a main terminal of a high-side switching element adjacent to a smoothing circuit (e.g., a source of a MOS transistor) is connected to a semiconductor element, such as a rectifier element forming the smoothing circuit or a low-side switching element (in a case of synchronizing rectification). In general, the semiconductor element includes a parasitic capacitance component (parasitic capacitance, stray capacitance). Thus, at a moment when the high-side switching element is turned on, a reverse current flows through the capacitance component, and the reverse current and a parasitic LC component in a circuit resonate. Accordingly, large switching noise is generated.
The above-described conventional technique does not consider reducing the switching noise generated due to the reverse current. Furthermore, the above-described conventional technique may increase the switching noise generated due to the reverse current. The reverse current is proportional to the capacitance value of the parasitic capacitance component and the voltage change amount of the output voltage of the switching element. Thus, when the output voltage drastically increases until the output voltage reaches the threshold value as the above-described conventional technique, because the voltage change amount increases, the reverse current increases and the switching noise due the reverse current further increases.
In other words, the above-described conventional technique increases the switching noise due to the reverse current that is a main factor of generation of switching noise in consequence of control for restricting overshoot that is not the main factor of generation of switching noise. Thus, when all the various factors considered together, the above-described conventional technique may increase switching noise rather than reduce switching noise.